A whipstock is a long, hardened steel wedge that forces a window mill to cut out through the side of casing to create a window. The smaller the angle change at the top of the whipstock, the larger the drilling assembly that can span across the angle change at top of whipstock, and so pass through the window. Smaller angles require longer windows and more metal to be cut. Whipstock angle is typically referred to as a measure of dogleg severity, but it is actually degrees of angle change per 100 ft. Since the angle changes abruptly at the top of whipstock, this measure is technically infinite. Instead it is assumed that the angle of the pipe changes over some distance as the pipe bends in a radius rather than at a point. Any design feature that makes the angle change more gradual will allow a longer, larger diameter drilling assembly to pass through. The two casing exit bottom hole assembly (BHA) design goals are maximum life and drilling assembly size.
A typical 3 mill window cutting BHA consists of an upper watermelon mill, a lower watermelon mill and a window mill. The window mill moves to the side as it progresses down the whipstock, which moves the lower toward the casing as well. Bending between the lower and the window mills starts when the BHA has rotated enough for the upper and the lower to contact the casing. These two contact points constrain the lower from further lateral movement (FIG. 7 below). The greater the clearance between the mills and the casing, the farther the window mill moves down the whipstock before the lower contacts the casing. After contact, the pipe between the lower and window mill is bent as the window mill continues to be forced to the side. The length and diameter of lower blades can further increase the bending. As the flex joint between the upper and lower is bent, the lower is inclined in the casing. If the lower diameter is large and the blades are long, the front and back of the blades can contact the opposite sides of the casing and cause the mill to lock up. After lock up, the inclination of the lower is fixed, so bending between the lower and the window mill increases rapidly. The lower should be dimensioned such that it is free to incline to the whipstock angle. The bending between the lower and window mills increases until lower moves onto the scoop, and so is greatest when the lower is at the top of the whipstock. When the lower spans the angle change at the top of the whipstock, it interferes with the casing at the top of the window, so the rotary torque increases at this point. Torque and bending are greatest when the lower is at the top of the whipstock.
If the BHA bending is excessive; the stress below the lower mill blades will be high enough to crack the body from fatigue. It is difficult for the window mill to first mill through the casing, and excessive bending further increases the force required. Before the window mill “gets out” through the casing, a large area of the window mill is bearing on the casing and a high side force is required for it to cut out. The force exerted on the window mill by the bent pipe between the window mill and the lower is in the opposite direction, so the force the whipstock must exert to cut out is increased by the amount required to overcome the bending force.
The most effective variable for controlling bending stress is the lower mill diameter. The smaller the diameter, the greater the clearance between the lower mill and the casing, and the farther down the whipstock the window mill will be before the lower contacts the casing and the lateral window mill movement starts bending the pipe. Reducing the lower diameter allows the designer to choose the point at which bending begins. The further the onset of bending is delayed, the lower the peak bending stress and the fewer of cycles of high bending stress will occur.
The principal function of the lower is to dress the top of the window. The “kink” in the wellbore path is at top of whipstock where the angle changes. In order to produce a low drag window, the lower should start cutting into the casing above the whipstock to provide clearance for long, large diameter drilling BHA elements. The lower needs some load against the casing for cutting, but excessive load wears the cutting structure quickly and can make the mill too smooth to cut effectively. There is a side load “sweet spot” where it is high enough to cut casing, but does not cause excessive torque, mill wear, pipe fatigue, and milling into the whipstock.
After the lower is on the scoop it is aligned with the window mill, and the bending between them is greatly reduced. However, there is now an angle change between the lower and the upper, and the bending of the flex joint is increased. The bending stress above the lower mill is less severe because the pipe between the lower and upper is much longer than between the lower and window mill. This bending pushes the window mill against the whipstock which causes it to stay in contact with the whipstock farther before it drills out into the formation. Note that side force between the window mill and the whipstock is not needed on the top half of the exit to keep the window mill against the whipstock because it is contained by the casing until the window width is equal to the window mill diameter.
Technically, the side force SF, for a bent pipe is given by the equation:
  SF  =            3      ⁢                          ⁢      E      ⁢                          ⁢              π        ⁡                  (                                    OD              4                        -                          ID              4                                )                    ⁢      d              8      ⁢              L        3            where:    E is the modulus of elasticity    d is the deflection    OD is pipe OD    ID is pipe ID    L is the length of the pipe
The side force increases as the fourth power of the pipe diameter and the third power of the length. So side force increases exponentially as the pipe diameter is increased and the length is shortened. For the same deflection, a 5″ diameter pipe has 8 times as much side load as a 3″ diameter pipe. A 3 ft pipe has 8 times as much side load as a 6 ft pipe.
If this restoring side force into the whipstock is too large, the bending stress on the lower will be excessive and it will fail in a short period of time. A high force will also increase window mill wear and the depth cut into the whipstock, which lowers the top of the window. Excessive bending force is to be avoided.
In a three mill casing exit BHA, the principal function of the lower mill is to cut additional casing away from the top of the casing exit window. The top of the whipstock is an abrupt angle change that causes long, large diameter drilling assemblies to bear against the top of the window when they span the angle change. The higher the top of the window, the larger the drilling assembly that can mount the whipstock without interference. Conventional lower mills come in two basic styles. The traditional style has a long full diameter section to reduce diametral wear. The objection to this design is a long length of cutting structure bearing on the casing slows the rate the mill cuts into the casing, which reduces the angle of the cut and lowers the top of the window (FIG. 1). The angle of the mill is exaggerated for clarity in the figure. The actual length of cutting structure engaging the casing is longer because the mill is only inclined about 2°. The long angle of engagement tends to wear a long taper on the cutting structure that further increases contact area.
The other mill style reduces contact area by making OD length short. The objection to this design is the after the initial cutters breakdown, there is no cutting structure left to continue cutting.
The present invention comprises a number of short OD surfaces to combine the aggressive cutting of a short OD surface with the longevity of a large number of cutters (FIG. 2). The number of rows bearing on the long angle cut in the casing is reduced by half for faster cutting, but a sufficient number of cutters are provided to complete the cutout and maintain the original mill OD. This will increase the angle of the cut and raise the top of the window. Shallow grooves are cut into the blades underneath the cutters to locate them when they are applied to the mill. As shown in FIG. 3, when the lead cutter breaks away, the following cutter will cut the same path again, but deeper. This also helps increase the cutout angle. The standard mill and the new cutting ring mill are shown in FIGS. 4 and 5. FIG. 4 shows the typical standard mill with Long OD surface and Glyphaloy® cutting structure where minor damage occurs at the leading edge while cutting above the whipstock and at the trailing edge while spanning the top of whipstock. FIG. 5 illustrates the cutting structure of the present invention.
The following references are relevant to some of the aspects of the present invention. CA2288494 (C); US2008/0093076 A1, U.S. Pat. Nos. 7,575,049 B2 and 7,370,702 B2.
Those skilled in the art will understand additional aspects of the invention by a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined by the appended claims.